Analysis of Halo Formation in a Dc Photoinjector

We discovered, by modeling the AES/JLab directcurrent photoinjector with several beam-simulation codes, that nominal injector settings would create a large diffuse beam halo as a consequence of the internal space-charge force in the beam. The injector-induced halo is sensitive to the injector settings, but if the settings are judiciously chosen, it can be largely circumvented. We present an exploration of the parameter space for the AES/JLab photoinjector. Measurement of beam halo will be a crucial aspect of commissioning this machine. INTRODUCTION The present driving accelerator for the Jefferson Lab 10 kW IR Free-Electron Laser (FEL) provides a 10 mA average current electron beam with energy up to 160 MeV and normalized transverse emittance less than 30 μm [1, 2]. Since the gain of the laser increases with the electron beam current [3], to achieve megawatt-class FELs the injector should be upgraded to increase the average beam current at ampere level. The normalized transverse emittance scales linearly with the laser wavelength and the relativistic factor γ [4]: